Wireless networks, including cellular telephone networks, have become ubiquitous in society. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is increased by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. To maximize usage of the available bandwidth in each cell, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Every cellular base station has an RF transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the RF power amplifier in a base station transmitter operate in a highly linear manner, especially when amplifying a signal whose envelope changes in time over a wide range, as in CDMA and multi-carrier systems. It also is important that the RF amplifier have good linearity characteristics across a wide range of operating conditions, because wireless systems cannot tolerate large amounts of signal distortion and may not violate adjacent channel power specifications, such as the IS 95 bandwidth requirements, regarding spectral spreading effects.
The output stage of an RF amplifier typically contains a high-power transistor, such as a class AB laterally diffused metal-oxide-silicon field-effect transistor (LDMOS FET), a gallium-arsenide (GaAs) FET, or, perhaps, a bipolar junction transistor (BJT). In order to maintain linear operation in the RF amplifier, the bias voltage of the output stage high-power transistor must be adjusted so that the bias current of the high-power transistor remains constant over a range of temperature.
For example, in an LDMOS FET, the gate-to-source bias voltage (V.sub.gs) must vary such that the quiescent current (I.sub.dq) remains constant as temperature rises. To maintain constant I.sub.dq over a temperature range, the gate voltage must decrease as temperature increases. The desired slope (mV/C) of the gate voltage varies from one device to the next due to process variation. If I.sub.dq is not constant over temperature, the device linearity or adjacent channel power ratio (ACPR) degrades. If the ACPR degrades, the RF amplifier output power must be reduced to the point at which it again complies with the J-STD-019 spectral mask. This reduction in output power decreases the overall range and capacity of cellular and PCS base stations.
One technique for biasing the output power transistor is to use a fixed-bias voltage. The fixed-bias approach is generally implemented with a simple voltage divider or adjustable reference voltage. Unfortunately, this technique is not capable of compensating the bias voltage over temperature, nor is it capable of compensating for lot-to-lot device variations. Furthermore, the fixed-bias technique is subject to thermal runaway. If the bias voltage is not temperature compensated, the bias current becomes very large with increased temperature. Under full RF drive conditions, the increase in bias current may become so large that the device overheats to the point of failure. Regardless of failure, the device mean-time-to-failure (MTTF) degrades with increased current and temperature.
Another technique for biasing the output power transistor involves the use of microprocessors and/or electronically programmable resistor arrays. This approach is much more complex and costly and requires input and output data from a master controller card. Furthermore, in order to measure and adjust the quiescent current, the RF input signal to the output power transistor must be temporarily shut off. Obviously, when the RF input signal is removed, the base station no longer transmits and all calls must be dropped. Thus, the base station must go out-of-service just prior to and during adjustment of the bias current.
There is therefore a need in the art for improved systems and methods of biasing the output power transistor of an RF amplifier to compensate for temperature variations. In particular, there is a need for temperature-compensated biasing networks for the output power transistor of an RF amplifier that are simple and inexpensive and that do not require that the base station be temporarily put out of service.